1. Field of the Invention
This invention relates to a process for removing the solvent from a polymer solution, more particularly to a process for removing the solvent from an elastomeric polymer solution which has a polymer content of at least 8% by weight.
2. Description of the Related Art
A high pressure steam stripping process and a direct solvent removing process have been employed in order to remove the solvent from an elastomeric polymer solution so as to form subsequently granules and crumbs of the elastomeric polymer.
In the high pressure steam stripping process, high pressure steam is used to remove the solvent from an elastomeric polymer solution. The high pressure steam is introduced into the elastomeric polymer solution in order to heat the elastomeric polymer solution and vaporize the solvent in the elastomeric polymer solution. Afterwards, a large part of the water, about 45% by weight, left in the elastomeric polymer solution is removed with the use of a water removing device, and the remaining water is subsequently removed with a degassing device, such as a single-screw extruder, a twin-screw extruder or a conventionally used expeller with an expander followed by a steam heated dryer, until a residual water content of less than 1% by weight is obtained. Since the polymer solution is heated indirectly by the steam, the problem of localized overheating can be eliminated, and the elastomeric polymer can be prevented from becoming a gel to avoid loss of the elastic characteristics of the former. For these reasons, the solvent in the elastomeric polymer solutions of polybutadiene rubber and polyisoprene rubber are generally removed by this method. However, the cost for removing a large amount of water with the use of this heating method is considerably high. Furthermore, a certain level of residual water, about 5-10%, will be left in the dewatered product of the elastomeric polymer. In order to remove the residual water in the dewatered product, a drying machine, which has a high electrical consumption, is required. In addition, all of the removed water must be treated before discharging. Thus, the high pressure steam stripping process is not an economical process.
A direct solvent removing process for removing the solvent from a solution of a polymer has been disclosed in Luxembourg Patent Application No. 86810. Referring to FIG. 1, the direct solvent removing process uses a solvent removing apparatus (B) which includes an extruder (92) that incorporates a twin endless screw (94). The elastomeric polymer solution, which has been preheated to 150.degree.-200.degree. C., is introduced into a feed hopper (90) of the extruder (92). The extruder (92) further comprises a rear ventilation vent (80) through which a large amount of the solvent, namely from 80 to 95% of the latter, will be removed. The polymer solution is driven forward by means of the endless screw (94) and is reheated simultaneously to a temperature of between 150.degree. C. and 200.degree. C. The heat input can be produced by heating the extruder barrel to a temperature of between 180.degree. C. and 250.degree. C., by providing the screw with components that introduce shearing forces or by a combination of both. Three ventilation vents (81, 82, 83) are arranged on the extruder (92). The remaining solvent is removed progressively through the various ventilation vents (81, 82, 83) in step with the forward movement of the polymer solution in the screw (94), while the vacuum increases in step with the progress of the polymer solution in the screw and changes from atmospheric pressure to approximately 5 mbars at the screw end. Between the ventilation vents (81, 82, 83), means (96) for introducing a fluid, generally water, has been provided to promote the removal of the solvent. The polymer solution, freed from its solvent in this manner, passes through a die (24') and is cut into granules. Therefore, a residual solvent content equal to or lower than 0.1% by weight is obtained.
In the above patent, the polymer is reheated by the screw barrel. The rate of heating is relatively low when the viscosity of the elastomeric polymer solution is high. In addition, the direct solvent removing process is not suitable for treating a heat-sensitive polymer. When the direct solvent removing process is used to treat polybutadiene rubber solution, which has a high viscosity, a long heating time is needed in order to heat the polybutadiene rubber solution to a temperature of between 150.degree. C. and 200.degree. C. in the twin-screw extruder (WP type ZSK-57). The prolonged heating results in crosslinking of parts of the polybutadiene rubber, thus causing a loss in the elastic characteristics of the latter. When such a partially crosslinked elastomeric polymer is used to modify an impact-resistant polystyrene, hard grains or fisheyes are formed on the surface of the resultant modified product. Aside from the formation of fisheyes, the polybutadiene rubber may further lose its modification effect when the polybutadiene rubber undergoes crosslinking. Therefore, the direct solvent removing process is not suitable for treating a heat-sensitive polymer with a high viscosity.
Another problem of the above patent stems from the design of the ventilation vents of the apparatus (B). A large amount of fine elastomer particles are entrained by the venting solvent, thereby clogging the ventilation vents. The removal of the clogging particles requires a shutdown operation, thereby lowering the efficiency of the apparatus (B). Due to the rapid vaporization of the solvent because of the reduced pressure at the ventilation vents, bubbles form rapidly in the elastomeric polymer, thereby resulting in the formation of fine elastomer particles. The bubbles in the elastomeric polymer produce fine elastomer particles upon breakage of the same. For example, the direct solvent removal process and the apparatus (B) are used to treat a styrene-butadiene-styrene block copolymer solution. When the styrene-butadiene-styrene block copolymer solution is moved forward in the screw (94) in order to remove the remaining solvent via the various ventilation vents (81, 82, 83), fine particles of the styrene-butadiene-styrene block copolymer are produced and block the ventilation vents (81, 82, 83). Thus, the operation of the apparatus (B) should be stopped frequently to maintain the ventilation vents (81, 82, 83).
U.S. Pat. No. 3,917,507 discloses a countercurrent combined liquid and vapor stripping process in a screw devolatilizer. This patent generally relates to a process and apparatus for removing volatile and soluble components from plastic material in a screw extruder. The screw extruder has a vent which is provided with a vapor removal means in order to prevent clogging of the vent. The screw extruder has a stripping zone which defines a high pressure downstream point, a low pressure upstream point and a stripping agent injection point that is located between the high pressure downstream point and the low pressure upstream point. The vent of the screw extruder is located in the stripping zone and upstream from the stripping agent injection point. The stripping agent is a liquid-gas mixture which flows in the screw extruder in a countercurrent direction with respect to the plastic material in order to remove the undesirable volatile components and the solvent from the latter via the vent. The vapor removal means in the port includes twin interlocking screws that are surrounded by a closely fitting housing with a clearance that is sufficient to permit only gases and liquids to pass therethrough. The twin interlocking screws are pitched and are rotated by a drive motor in a direction so as to force entrained solids back toward the plastic mass in the screw extruder. Therefore, the port is prevented from being clogged. It is noted that the screw extruder which incorporates the twin interlocking screws is similar to a pair of intermeshing co-rotating twin screw extruders which are fed transversely in a second pair of twin screws in order to produce devolatilization and which have been taught in German Patent No. 915,689 by R. Erdmenger et al. In the U.S. Pat. No. 3,917,507, the screw extruder has a housing that is surrounded by a heating jacket. Because the polymer material in the screw extruder is heated by means of the heating jacket, the heating time needed for heating the polymer material to a predetermined temperature is relatively long if the shear force heat is not effective. Therefore, the apparatus in the U.S. Pat. No. 3,917,507 is only suitable for treating plastics which have a low volatile component content, such as ABS polymers (acrylonitrile-butadiene-styrene), and is not suitable for treating an elastomeric polymer solution and a material which has a high solvent content, even though the port has been designed to prevent clogging.
Accordingly, the main problems of the above described conventional solvent removing processes reside in that fine particles of the elastomeric polymer easily clog the ventilation vents, and crosslinking of the elastomeric polymer solution, which has a high elastomeric polymer content and a high viscosity, occurs easily due to overheatinq, thus causing the elastomeric polymer to lose easily its elasticity.